1. Field of the Invention
The present invention generally relates to wireless communication systems, and more particularly to methods for generating pilot beacons in such systems.
2. Description of Related Art
A spread spectrum mobile communication system (e.g., CDMA) typically includes mobile stations that are capable of moving around unspecified regions, and base stations for receiving/transmitting messages from/to the mobile stations, as well as managing radio resources. A control station may be provided for controlling the base stations and the reception/transmission of messages from/to the base stations, and a switching center may provide connections to the control station, switching centers of other communication systems, or other communication networks, for communication between mobile stations or between the mobile station and a fixed station. As an important feature of a spread spectrum mobile communication system is assurance of mobility for a subscriber's mobile station, handoff technology is required.
A handoff is a method of switching a communication path between the mobile station and a base station so that communication is continuous when the mobile station goes out of the coverage area of a first base station and into the coverage area of a second base station. A handoff may also be used when the mobile station moves beyond a service region of a sector, or between cells, for example.
There are typically three (3) types of handoffs: a softer handoff, a soft handoff, and a hard handoff. The softer handoff occurs when the mobile station moves between sectors of a single base station. In this type of handoff, a frequency and a frame offset are maintained, and the mobile station sets up a new communication channel with a destination sector, while the present communication channel between the mobile station and the base station is maintained. Thus, a packet, such as a voice or data packet, may be transmitted on many channels. The initial communication channel is subsequently cut off when the mobile station has sufficiently moved into the destination sector such that maintenance of the initial communication channel is no longer required.
The soft handoff may occur when the mobile station moves between adjacent base stations while a frequency and a frame offset are maintained. In this handoff, the mobile station sets up a communication channel with a destination base station while maintaining the initial communication channel with the first base station. A packet, such as a voice or data packet, may be transmitted on many channels. The initial communication channel is subsequently cut off when the mobile station has sufficiently moved into the destination cell such that maintenance of the initial communication channel is no longer required.
A hard handoff may be used when the mobile station moves to an adjacent base station and the frame offset or frequency is changed. Additionally, the hard handoff may be used when the mobile station moves to another switching center. Unlike the softer handoff and the soft handoff, the mobile station cannot maintain the initial communication channel. Rather, a newly set up communication channel is used to transmit the voice/data packet.
With the advancement of technology and the demand of the market, it is foreseeable that more private spread spectrum networks will be installed in addition to existing public spread spectrum networks, such as the Sprint PCS CDMA network by Sprint, Inc. Major corporations or institutes may have the desire to establish private spread spectrum networks on their corporate/institutional sites or grounds (hereafter “sites”) to provide higher quality and/or more economical wireless service to their employees or guests. However, such sites are often heavily populated with mobile units such as cellular phones, hand-held PCs, PDAs, etc., for example. To worsen the situation, the size of the site may be relatively small, so that it may reside entirely within one cell of a larger network.
A feature of spread spectrum technology that differentiates it from other wireless communication technologies is its use and reuse of specific frequencies. Although this may provide a technical edge over other communication technologies such as time division multiple access (TDMA), reuse of specific frequencies makes the corporate site scenario described above undesirable, since a comparatively large number of users may have to be serviced by one sector of the cell using the same frequency. This may reduce the quality of the wireless services provided to the mobile units on the site.
As a result, it may be desirable for a private spread spectrum network on a site to use a carrier frequency different from the surrounding networks. Consequently, signal interference between the surrounding network and the private network on the site may be greatly reduced because different carrier frequencies are employed. However, for providing continuous wireless service to a mobile station when it enters or leaves the site, a hard handoff must occur between the private network and outside network(s).
There are several difficulties associated with hard handoff. An active wireless service, such as an ongoing telephone call, should not be interrupted or dropped due to the change of the carrier frequency when carrying out such a hard handoff. In addition, when a mobile user exits the site, he should be able to continue his conversation on the mobile station, even though the mobile station needs to switch from the private network to the surrounding network. Such a transition should be performed in a relatively small border area between the site and outside, and vice versa when the mobile user enters the site.
Unfortunately, various problems exist for performing a handoff at the border area. Soft handoffs would not work well in this situation. In order to take advantage of a soft handoff, only one carrier frequency can be involved. Soft handoffs are thus most useful for switching wireless service between two base stations using the same carrier frequency. For the situation discussed above, since at least two carrier frequencies would be involved, the soft handoff scheme would be difficult to deploy.
For example, in order to use a soft handoff scheme, as it is known in the art, the only alternative is to configure a tier of cells for providing a soft handoff zone between the site and the surrounding network. This would dramatically increase the cost of installing such a private network since additional, expensive infrastructure equipment must be installed. Moreover, significant redesign of the surrounding network is also required to accommodate this change, which may add further costs to the private network.
Therefore, a soft handoff may be economically infeasible. A hard handoff design may therefore be more preferable. But there are problems with using a conventional hard handoff scheme, such as a round trip delay handoff in a small area. As it is known in the art, the round trip delay handoff can detect the motion of a mobile station only when it moves for more than 250 meters. This may not work for the site scenario mentioned above, because the sites themselves may have a radius of less than 500 meters. The handoffs thus cannot be guaranteed to happen at or near the entrance, or the exit, of the site, since the border area between the site and the surrounding network(s) may only be about 25 meters in length, for example.
However, the above problems with hard handoff are being addressed by installing pilot beacon generators at entrances/exits of a site. For example, assume that an entrance to a site, or private CDMA network, is surrounded by an existing public CDMA network. The private CDMA network has a carrier frequency F1 (i.e., operational frequency), and the surrounding CDMA network has a different carrier frequency F2.
For example, two pilot beacon generators, also known as pilot beacon distributors, may be installed at or near entrances of the site, where at each entrance one inbound pilot beacon distributor directs handoff to the frequency F1 of the private network, and one outbound pilot beacon distributor directs handoff to the frequency F2 of the public network. The pilot beacon distributors may contain beacon antennas for transmitting pilot beacons. Thus, once a mobile station passes through an area covered by an outbound pilot beacon distributor, its wireless service is expected to be switched to the frequency F2 of the surrounding public network. Similarly, if the mobile unit passes through the area covered by the inbound pilot beacon distributor, it is expected to be switched to the frequency F1 of the private network on the site.
In a complex communication network, multiple carrier frequencies may be employed both for the private network and the surrounding networks. Since a pilot beacon is needed for each carrier frequency at each location, this limitation may greatly and rapidly increase the cost of a pilot beacon generation system when networks expand to more carrier frequencies.
However, conventional pilot beacon generation or distribution systems assume that when a plurality of pilot beacons are deployed, all of them carry identical information being transmitted at different frequencies. For example, an original pilot beacon at one frequency F may be split into N copies of the pilot beacon S0, S1 . . . Sn. These copies, except S0, may be further fed into a mixer that creates intermediate signals at slightly different frequencies around the original frequency F. Subsequently, a band pass filter may be used to eliminate unwanted image frequencies to render a resultant pilot beacon at a distinct carrier frequency. Thus, the resultant set of pilot beacons F1 . . . Fn, in conjunction with S0 (which is now F0), have different frequencies that will not interfere with each other.
But, in fact, each of the pilot beacons may not carry identical information being transmitted at different frequencies. For example, conventional pilot beacon generator systems do not actually determine an unknown pilot beacon signal, they simply generate a plurality of guesses (e.g., set of pilot beacons F1 . . . Fn, at different frequencies so as not to interfere with each other). Conventional pilot beacon generator systems may not account for a scenario where two neighboring CDMA systems, or neighboring cells along a border have the same number of operational frequencies, (or carrier frequencies), but which are not be at identical frequencies (e.g., cell A has carrier frequencies at F1 and F2, cell B at F3 and F4); or alternatively, two neighboring systems and/or cells that may have the same operating frequencies, but one of the bordering communications systems or cells may be transmitting an additional operational frequency over an additional channel (e.g., cell A has one channel at F1, cell B has a channel operating at F1 and a channel operating at F2)). Further, the conventional pilot beacon generation systems do not provide a mechanism to inform a base station in the private (or public) network whether or not the pilot beacon generation system is functioning properly, so as to continuously generate pilot beacons.